The motor draws inductive current when 50 HP induction motor is run by feeding three phase AC supply. At no load running of the motor major part of the current is inductive in nature called magnetizing current. When motor is run on load, the motor draws inductive current. The inductive current drawn by the motor increase the heat loss in the cable and in the motor winding. For reducing the line current, the capcitor is connected across the motor winding to compensate the reactive current. Required capacitor KVAR for 50 Hp motor to compensate inductive current can be done as follows.
First calculate the reactive KVAR of the motor operating under no load.
KVAR of the Motor =√3 VL IL Sinθ
Where, VL and IL is the line voltage and the line current respectively. The angle θ is the phase angle between voltage and current.
The line current is the vector sum of the current drawn by the capacitor and motor.
Required capacitor KVAR for 50 HP motor to nullify the effect of inductive KVAR of the motor must be just close to inductive KVAr of the motor.
50 HP = 0.745 x 50 = 37 KW
The motor draws about 1/3 rd of its Full Load Current under no load condition. The capacitive KVAR required to compensate magnetizing current of the motor under no load condition is;
Required Capacitor KVAR for 50 HP Motor
=1/ 3 * Motor rated KW
= 1/3 * 37
If installed capacitive KVAR is higher than the running inductive KVAR, this situation may lead to leading power factor that will increase the line voltage. The increased voltage may break down the motor insulation.
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